Diabetes mellitus and cognitive decline are major public health concerns among the elderly. In diabetic subjects without dementia, certain cognitive domains are impaired, such as memory, attention, and executive/frontal lobe function (diabetic cognitive dysfunction). Recent epidemiological studies have suggested that diabetes increases the risks for vascular dementia as well as Alzheimer's disease. There are accumulating evidences that indicate biological linkage between impaired brain glucose metabolism homeostasis and cognitive decline. Diabetes may cause serious brain damages through several mechanisms and induce a variety of cognitive decline. Most critical issue to be resolved is to identify the mechanism of dementia leading from diabetic cognitive dysfunction. Once elderly diabetics had severe cognitive decline, effective treatment of diabetes were hardly obtained. Thus, diabetic cognitive decline should be considered as an important comorbidity of the elderly diabetes and long-term management of hyperglycemia is required from a view point to sustain healthy brain function. In this short review, we are summarizing the clinical features and current biological findings of diabetic cognitive decline. Also, we introduce the comprehensive treatment of demented diabetic elderly, including therapeutic strategy, nursing and care.